This invention relates to a method and apparatus for manufacturing metal pipe and more particularly to a cold forming method and apparatus for manufacturing submerged arc welding pipe using a skelp or a plate as the blank.
Heretofore, in manufacturing submerged arc welded pipe from a skelp or plate, for example, in the so-called UOE system of forming a large diameter pipe, a plate initially has both edge portions bent to form the plate into a shallow wide trough by being pressed between a top die having convex curved surface and a bottom die having concave surface.
Next, the center portion of the plate is bent in almost a U-shape cross-section by a top U-shape punch of the single cylinder type or double cylinder type and a pair of rotary rocker type dies disposed on the left and right sides of the plate which cooperate to folding the plate with a U-shape cross-section blank.
Thereafter, the U-shaped blank is fed into the space between top and bottom dies defining an O-shape and the gap between the top and bottom dies is gradually reduced whereby the blank is shaped as it moves along the inner surface of the dies and finally compression is applied in the peripheral direction of the blank so that the blank is formed into an O-shaped cross-section and then the inside and outside surfaces are welded at the abutting edges of the blank and the thus formed cylinder is expanded to become a finished pipe.
In the conventional forming method, the following problems arise.
(1) In general, the bending into the U-shape can be carried out with relatively small force, but during the shaping into the O-shape cross-section, the cross section of the blank becomes polygonal in the initial period of the forming, and when forming a high tension and large wall thickness steel pipe having a plate thickness greater than 35 mm, with presently available press capacity (50,000-60,000 ton), forming of the polygonal shape into a round shape is difficult. Particularly, the area around the longitudinal edges is hard to shape into good roundness, and this deficiency cannot be corrected sufficiently during the succeeding expanding step.
(2) In general, in the shaping into the O-shaped cross-section, where the compression is applied in the direction of periphery of the pipe, the load P necessary for obtaining a satisfactory shape is about 2-3 times the load applied at the moment of full contact of the blank and the inner surface of the die as will be described hereinafter, and it is represented by the following equation, EQU P= 1.15 6y.2t.1 (1)
where .delta.y: yield strength of the blank (kg/mm.sup.2) PA1 t: plate thickness of the blank (mm) PA1 l: length of the blank (mm)
Since this load is proportional to the plate thickness and pipe length and yield strength of the blank, it becomes extremely large if the plate thickness and the length of the pipe become large, and in case of large wall thickness high tension steel pipe having a yield strength of 52 kg/mm.sup.2 and a plate thickness of 40 mm and a length of 18 m, the load required for the forming is 86,000 tons which requires a huge press forming machine.
Also, in forming a long heavy gauge pipe, after the edge bending and rough U-shaped forming by a press machine having a relatively small capacity, forming by the O-shape die which has a short length takes place, but the shape is not necessarily stabilized, and in order to prevent die marks which tend to occur in the forming process, a tremendous amount of time and a number of forming steps are required making it extremely difficult to manufacture pipe economically.